1. Field of the Invention
The subject of the invention is a fast acting electromagnetically actuated miniature valve which is specifically useful for applications in automobile technology. The valve is controlled by means of state of the art electronic circuitry with frequencies of up to several hundred Hertz using variable length drive pulses. It is suitable for pressure ranges up to approximately 200 bar. The cross-sectional area of flow is from 0.5 to 10 square millimeters. The principal application area is the electronically controlled pressure regulation in automatic transmissions. Additional application areas are to be found in such cases where especially high requirements exist for high speed switching action, repeat precision, and durability. Among these, the following applications are examples: pilot control of Diesel injection nozzles, power steering, automatic anti-skid systems, or electronic suspension tuning.
2. Problem Definition and State of the Art
In order to improve driving comfort and reduce energy consumption, the automotive industry is striving toward the electronic control of switching processes in automatic transmissions. This requires exact control of the pressure variables in the individual hydraulic modules of the transmission. The electronic valves required for controlling the pressure variables can be divided into two main categories: analog controlled valves and pulse modulated valves.
Analog controlled valves are set by changing the strength of the electrical current. The necessary control circuitry is relatively costly and complex. The magnetic circuitry of analog controlled valves can only exert small displacement forces. Even minor changes in the actuating force requirements can cause considerable deviations from the rated output of the valve. Therefore, these valves are sensitive to even the slightest tolerance changes. They are sensitive to changes in oil-throughput, viscosity, and contamination of the hydraulic oil. In addition, valves of this type exhibit a considerable hysteresis. Manufacture of such valves, as well as their calibration and the necessary quality control are costly and labor intensive. Despite the listed disadvantages, at present only the analogtype valves are capable of meeting the requirements of the automotive industry. Thus, to date, only analog valves have been adopted in mass production.
Because of the disadvantages detailed above for the analog-type valves, especially the American automotive industry is desirous of using pulse modulated valves. Such pulse modulated valves control the desired pressure variables by changing the control factors. To this effect the hydraulic load is alternately connected, by means of a three-way valve arrangement, with the pressurized oil source and an essentially unpressurized return oil stream. This effect is normally achieved at constant frequency, but at variable duty cycles of the electrical current. By means of the resulting pressure pulses, and given a high enough frequency of the events, the desired average pressure at the load section is achieved. This digital process offers considerable advantages, both with respect to energy consumption and controllability, over the conventional analog procedure.
In principle, such pulse modulated control devices have been known for some time. An introduction to this technology can, for instance, be found in the article published in 1972 by Hesse and Moeller (Pulse Duration Modulated Control of Magnetic Valves; Oelhydraulik und Pneumatic 16, specifically on page 451).
Despite the known advantages of pulse modulated pressure control, this technology has to date not found any entry into mass production. The reason for this is mainly found in the heretofore inadequate durability of the devices, and the often deficient speed of switching action of the valves which have been used in experimental trials. The special requirements of the pulse modulated operating procedure shall be detailed in the following.
Pulse modulated valves for pressure control in automatic transmissions are employed at frequencies of 30-100 Hz. This frequency range was found to be required in order to provide for adequate transition speed from one state to the next, and also to provide for adequate decoupling between valve and hydraulic load element. For automatic transmissions with pulse modulated control, two different types of design are in use: one type involves direct activation of the displacement cylinders by pulse modulated valves; the other type involves pilot control of the pressure level in the total hydraulic system and action on the cylinders by means of simple three-way valves. In the direct action mode the valve is only pulsed during the shifting action. The lifetime required for on-line control is in the order of 10.sup.7 cycles, and the necessary cross-sectional area of flow is 0.5-10 mm.sup.2. In the pilot control design, the pulse modulated valve is in continuous use during drive operation. Because of the enormously high number of action cycles, the automotive industry in this case requires a lifetime guarantee of at least 10.sup.9 cycles. On the other hand, the pilot control design mode only requires a cross-sectional area of flow of approximately 1-2 mm.
It has been determined that for adequate reproducibility of the switching events for the intended application, pick-up and release times of less than 2 ms are necessary. These pick-up and release times should be achieved by means of the usual supply voltage of 12 V with maximum energizing currents of preferably less than 4 A, in addition these times should only vary insignificantly during actual use of the valve. To achieve such high-speed switching action is problematic, especially for direct on-line mode of operation, because of the relatively large cross-sectional flow requirements.
The general problems inherent in pulse modulated pressure control are further complicated in automotive applications by the special operating requirements. For these applications the valve has to operate reliably at temperatures as low as -40.degree. C. By reliable operation is to be understood that the valve must be capable of opening and closing actions at this temperature. Maintenance of rated specifications is required down to -20.degree. C. At -40.degree. C. the hydraulic oil has turned into a viscous, jelly-like mass, which, for most conventional valve designs, no longer provides for adequate lubrication. In addition the hydraulic working oil will contain abraded ferritic magnetizable particles which in certain designs tend to deposit at the working gap of the electromagnet and thus degrade the capacity of the magnet.
In the case of pulse modulated three-way valves two different hydraulic connection methods are possible, one where the load element in the non-energized state of the electromagnet is either connected to the pressurized oil source, or with the virtually pressureless return oil flow. The connection method used depends on the valve position required in the case of operating trouble. Under practical conditions the required connection method is, however, almost always the one where the load element is connected to the pressurized oil source in the case of the non-energized state of the electromagnet. With this connection method, the reset position of the valve is obtained via the supply pressure. This makes it possible to omit the reset spring which is normally required. It is to be noted, however, that this involves a greater sensitivity of the rated capacity to variations in the supply pressure. For extreme precision requirements with respect to the rated capacity range, even for this connection method, it is better to provide for a reset spring.
With respect to manufacturing requirements the automotive industry also has special demands. It is a matter of course that a suitable valve must be at least equivalent in cost and capacity rating to a comparable valve of analog design. In addition, there are the requirements for very small dimensional parameters and adaptability to the existing hydraulic conduits, so that the valve can be fitted into the limited spaces of already existing automatic transmissions. The external dimensions of the valve should approximate those of the conventional low pressure injection valves. In addition, a series model should be adaptable to the previously detailed hydraulic connection methods, and the various applications as found in general automotive hydraulic applications. This allows for a larger volume of basic series production and simplifies production logistics. As a natural consequence, production- and quality control-costs are lowered.
In summation, a suitable valve for pulse modulated pressure control of automatic transmissions must live up to the following requirements:
virtually wear-resistant operation, lifetime up to 10.sup.9 cycles, PA1 pick-up and release times preferably less than 2 ms, stable and reproducible performance, PA1 control directly by means of the supply voltage of 12 V, with peak currents of preferably less than 4 A, PA1 insensitive to oil contamination, PA1 operating range to -40.degree. C., PA1 short time dry-running capability, PA1 a series production model should be suitable for a number of applications, PA1 low cost item, suitable for volume series production PA1 Armature and obturator are a single solid unit, preferably manufactured out of one piece, and with a total mass of only a few grams. PA1 The valve stroke is significantly less than 0.5 mm, preferably 0.05-0.2 mm. PA1 The bearing arrangement for the valve obturator is such that a radial clearance of less than 0.05 mm is involved and the valve obturator is movable in the direction of the axis, and this bearing arrangement simultaneously guides the armature and also serves to separate two spaces of differing pressure. PA1 The valve obturator coordinates with two alternately closing valve seats. PA1 The seating surfaces of the armature, in both directions of movement, are exclusively defined by the valve obturator. PA1 The seating surfaces of the armature are perfused by the main oil stream prior to the armature reaching its final position. PA1 The valve seats feature approximately the same radius as the armature bearing, where the average radius of the valve seats does not deviate by more than .+-.1 mm from that of the armature bearing, and where this deviation of the radii of the valve seats preferably shall not exceed 0.4 mm.
The above application profile can only be partially met by the conventional valves which have been used experimentally. These conventional valves are usually modifications of the state of the art ball and socket valves or slide valves. The design features of the ball and socket valve are shown in FIG. 10, those of the slide valve in FIG. 11. Analysis of the design characteristics shows that the ball and socket valve will require either high magnetic forces, because of the non-pressure equalized surfaces, or, for the case of small diameter balls, relatively large armature strokes. The slide valve design features the advantage of completely equalized pressure surfaces, but due to the necessary coverage of the leading edges, also requires a relatively large armature stroke of at least 0.4 mm. Because of the relatively large armature stroke, poor electromagnetic efficiency would result for the dimensions required for pulse modulated valves, and the kinetic energy of the moving parts would be relatively high. High kinetic energy results in increased wear and difficult to control armature chatter. For the above reasons, the presently used experimental valves of ball and socket design, or slide valve design, which are under test by the industry for pulse modulated pressure control, are still less than adequate, especially with respect to reliability under stressed conditions, and also with regard to durability.
Analysis shows that the necessary operating requirements can only be met by electromagnetic valves with very low armature stroke, and very small armature mass. Only with stroke dimensions in the preferred order of magnitude of about 0.1-0.2 mm, and armature masses of only a few grams can the demanded short floating times be achieved. It is a further known fact, that for better efficiency use of the magnetic force in electromagnetic valves, an adaptation to steady state characteristics is desirable. By this, it is generally understood that the sum of the mechanical and hydraulic counterforces at the initiation of armature movement should be considerably lower than counterforce magnitude at the termination of armature movement. By means of such a synchronization of the mechanical and hydraulic counterforces a good fit to the powerflow of the electromagnet is achieved, resulting in a considerable reduction in floating times.
However, it is to be noted that special electronic circuits are known where the exciting current is reduced after initial armature movement. In this case, the sum of the mechanical and hydraulic counterforces must be lower than the maximum force reduced by the special circuitry after anchor movement, so that a premature anchor drop-back is avoided.
This type of adaptation is usually achieved by a combination of several spring systems or by means of very stiff spring characteristics. Such systems have been proposed by the applicant in previous German applications (P 33 14 899, P 34 08 012). Systems which rely on mechanical means of adaptation are, however, rather problematic because of the required manufacturing precision. Systems with mechanical adaptation features are poorly suited for mass production of valves with very small armature stroke.
It is the objective of this invention to devise a fast-acting hydraulic three-way valve in miniaturized form which is characterized by small armature stroke and adaptation characteristics according to the requirements stated above. The valve can be operationally used in the 0.5-10 mm.sup.2 cross-sectional flow range.